JP6607463B2 - Strain-induced aging strengthening in dilute magnesium alloy sheets - Google Patents

Description

Detailed Description of the Invention

[Technical field]
[1] The present invention generally relates to a method for strengthening a dilute magnesium alloy sheet using a strain-induced aging process. The invention is particularly applicable to sheets formed from magnesium alloys containing small amounts of zinc and calcium / rare earth elements, and it is convenient to disclose the invention below in connection with exemplary applications. Good.

[Background of the invention]
[2] The following discussion of the background of the present invention is intended to facilitate understanding of the present invention. However, it is understood that this discussion is neither an admission nor an admission that any material mentioned is known or part of the common general knowledge published at the priority date of the application. .

[3] Magnesium (Mg) is one of the lightest commercially available structural materials. Mg has a density of 1.74 g / cm ³ at 20 ° C., this characteristic, magnesium, automotive, aircraft, aerospace, and 3C (computer, communications, and consumer electronics products) structures, such as industrial applications To be a promising candidate. However, the room temperature formability of magnesium alloys is generally not high, which limits their large scale use.

  [4] Alloying can improve the ductility and formability of the Mg alloy. For example, Applicant's co-pending provisional application states that magnesium additions to magnesium-zinc type alloys can improve the ductility and formability of sheets formed from these alloys. -Relating to zinc-type alloys. Nevertheless, the addition of small amounts of alloying elements does not effectively strengthen the resulting alloy sheet. Therefore, it is desirable to further enhance the strength of the sheet formed from this type of magnesium alloy.

  [5] We are aware of several announcements reporting that high strength magnesium alloys can be formed using high concentrations of alloying elements, often near 10 wt% or higher. doing. These alloys often contain large amounts of zinc or one or more rare earth elements such as gadolinium, yttrium, neodymium, and cerium. Inclusion of these alloying elements results in precipitation hardening of the Mg alloy through the formation of a number of strengthened precipitates after the aging treatment, thus improving the strength of these dense Mg alloys. In contrast, magnesium alloys with dilute alloying compositions (<3 wt% total alloying composition) have not previously been considered to have similar age hardening responses. A small amount of alloying composition is believed not sufficient to produce the necessary strengthening precipitates. Thus, such dilute magnesium alloys and sheets made therefrom are not expected to have any significant age hardening response.

  [6] Accordingly, it is desirable to provide a method for producing a high strength magnesium alloy sheet formed from a dilute magnesium alloy.

[Summary of Invention]
[7] The present invention relates to a method for strengthening a diluted magnesium alloy sheet. A dilute magnesium alloy sheet is understood to be one or more sheets formed from a magnesium alloy containing less than 3 wt% alloying elements.

[8] The present invention provides:
(A) (wt%):> 0 to 3.0 Zn,> 0 to 1.5 Ca, 0 to 1.0% Zr, 0 to 1.3 rare earth elements or mixtures thereof, 0 to 0 Preparing a dilute magnesium alloy sheet comprising a magnesium alloy consisting essentially of 3 Sr, 0-0.7 Al, the balance Mg and other inevitable impurities, wherein the total weight percent of alloying elements is A step that is less than 3%, and
(B) forming a pre-deformed magnesium alloy sheet by plastically deforming the diluted magnesium alloy sheet such that the tensile plastic strain is greater than 0.5% but less than 8%;
(C) aging the pre-deformed magnesium alloy sheet at a temperature range of 80 to 250 ° C. for at least 1 minute;
Including
Thereby forming a reinforced magnesium alloy sheet,
A method for strengthening a dilute magnesium alloy sheet is provided.

  [9] Magnesium alloy sheets formed from alloys containing small amounts of zinc and calcium / rare earth elements, despite having excellent ductility, their strength is generally not high and they are not effectively aging strengthened . The inventors have improved (ie increased) the strength of these dilute magnesium alloy sheets, and more particularly the yield strength, by introducing a small amount of plastic deformation into the formed alloy sheet followed by aging treatment. ) And found that in many cases it can be greatly improved. Thus, the strain-induced aging strengthening phenomenon of the present invention is effective in strengthening these magnesium alloy sheets having a high degree of ductility and formability and allowing these alloy sheets to have more commercial value. Provide a means.

[10] The increase in strength due to the double treatment regime in which plastic deformation is followed by aging treatment can be measured by% increase or MPa increase. The overall yield stress depends on more factors than the processing method, factors such as sheet forming process, rolling conditions, annealing temperature of the process, and other factors that affect the properties and microstructure of the magnesium alloy sheet It is understood that it depends on factors. However, if the above considerations are constant, the strength increase or enhancement rate of the treatment process of the present invention can be quantified as follows.

In the formula, Y. S. Means yield strength.

[11] In some embodiments, the reinforced magnesium alloy sheet (SMA) reinforcement rate or strength increase (ie, 100 × (YS _SMA ) relative to the dilute magnesium alloy sheet (ie, as annealed after forming) (OMA). -YS _OMA / _{YS OMA)} is at least 10%, preferably. preferably at least 20%, increase in the strength of the reinforcing magnesium alloy sheet against dilute the magnesium alloy sheet, between 20% and 150%, more preferably It is understood that an increase in strength between 20% and 130%, between 20% and 100%, and more particularly above 50%, is a very unexpected result for dilute Mg alloy sheets.

  [12] At MPa, the strength increase of the reinforced dilute magnesium alloy sheet relative to the dilute magnesium alloy sheet (ie, SMA yield strength-OMA yield strength) is preferably at least 10 MPa, more preferably at least 20 MPa, and even more preferably at least 33 MPa. It is. In some embodiments, the strength increase of the reinforced magnesium alloy sheet relative to the dilute magnesium alloy sheet is between 33 MPa and 139 MPa, preferably between 35 MPa and 135 MPa.

  [13] The strength of the reinforced magnesium alloy sheet arises from the dual processing regime of the plastic deformation (or pre-deformation) step and the aging treatment step. The parameters and conditions of these steps are preferably controlled to optimize the strength of the resulting reinforced magnesium alloy sheet.

  [14] In the present invention, a small amount of plastic strain is used to cause a significant improvement in the strength of the dilute magnesium alloy sheet. For example, the tensile plastic strain from plastic deformation will be greater than 0.5% but less than 8%. In a preferred embodiment, the tensile plastic strain is controlled in the range of 0.5-6%, preferably 0.7-5%, more preferably 1-4%.

  [15] Further, the aging treatment will be performed at a temperature range of 80-250 ° C. for at least 1 minute. In a preferred embodiment, the temperature range for the aging treatment is between 100 and 250 ° C, preferably between 100 and 200 ° C. Similarly, in some embodiments, the aging treatment is 24 hours or less, preferably up to 12 hours. Furthermore, in some embodiments, the aging treatment is preferably for at least 5 minutes. For example, in some embodiments, the aging treatment is between 5 minutes and 12 hours.

  [16] The plastic deformation and aging treatment steps can be performed using any suitable apparatus and / or instrument. In some embodiments, the plastic deformation is achieved by at least one of tensile drawing or cold rolling. However, it is understood that other plastic deformation techniques such as bending and molding may be used. The tensile stretching is preferably performed at room temperature. Furthermore, if cold rolling is used in this step, the thickness reduction of the magnesium alloy sheet due to cold rolling does not exceed 20%, preferably does not exceed 15%, and more preferably exceeds 10%. Absent. Similarly, in some embodiments, the aging treatment is performed in air or oil, preferably in an oil bath. However, it is understood that other instruments can be used as well to provide the same processing.

  [17] The dilute alloyed composition of the magnesium alloy in the dilute magnesium alloy sheet is an important element of the present invention. In the most general compositional composition, the magnesium alloy is (wt%):> 0 to 3.0 Zn,> 0 to 1.5 Ca, 0 to 1.0 Zr, 0 to 1.3. It consists essentially of rare earth elements or mixtures thereof, 0-0.3 Sr, 0-0.7 Al, the balance Mg and other inevitable impurities, the total weight percent of alloying elements being less than 3%. In some embodiments, the magnesium alloy comprises 0.1-3.0 wt% Zn, preferably 0.5-2.0 wt% Zn. In some embodiments, the magnesium alloy comprises 0.05 to 1.5 wt% Ca, preferably 0.1 to 1.0 wt% Ca.

[18] Depending on the alloy composition, a certain amount of rare earth element may be present. In the most common form, the magnesium alloy comprises (wt%): 0 to 1.3 rare earth elements or mixtures thereof, but in some forms the rare earth elements or mixtures thereof are 0.05 wt% and 1 .3 wt% may be included. The rare earth elements or mixtures thereof in the magnesium alloy (if applicable) may include lanthanide series and / or yttrium rare earth elements. For the purposes of this specification, lanthanide elements include a group of elements having atomic numbers included in increments from 57 (lanthanum) to 71 (lutetium). Such elements are called lanthanides because the lighter elements in the series are chemically similar to lanthanum. Strictly speaking, lanthanum is a group 3 element, and the ion La ³⁺ does not have f electrons. However, for the purposes of this specification, it will be understood that lanthanum is included as one of the lanthanide series rare earth elements. Therefore, the lanthanide series rare earth elements include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. For the purposes of the present invention, yttrium will also be encompassed by the term “rare earth element”. In some embodiments, the rare earth component comprises gadolinium (Gd). In other embodiments, the rare earth component comprises a mixture of gadolinium (Gd) and lanthanum (La). In other embodiments, the rare earth component comprises a mixture of gadolinium and yttrium. An advantage of embodiments comprising the lanthanide series or yttrium rare earth elements is their relatively high solubility in magnesium.

  [19] In some embodiments, the magnesium alloy of the magnesium alloy sheet is (wt%): Zn:> 0 to 3.0, Ca:> 0 to 1.5, Zr: 0 to 1.0, Gd : 0 to 1.0, preferably 0.05 to 1.0, Sr: 0 to 0.3, La: 0 to 0.3, Al: 0 to 0.7, and the balance Mg and other inevitable impurities The total weight percent of alloying elements is less than 3%.

  [20] In a preferred embodiment, the magnesium alloy comprises (wt%): Zn: 0.1-2.0, Ca: 0.3-1.0, Zr: 0.2-0.7, Gd: 0 0.1 to 0.5, Sr: 0 to 0.2, La: 0 to 0.2, Al: 0 to 0.5, and the balance Mg and other inevitable impurities, The total weight percent is less than 3%.

[21] The present invention can be applied to the strengthening of highly formable magnesium dilute sheet alloys, but Mg- (Zn) -RE, Mg-Zn- (RE) -Ca-Zr and Mg- It can also be applied to extruded magnesium products made from Ca-Zn- (Zr) compositions. Accordingly, in some embodiments, the particular magnesium alloy used in the magnesium alloy sheet can be classified into the following three general dilute magnesium type alloy compositions.
Group 1: Mg- (Zn) -RE type alloys,
Group 2: Mg-Zn- (RE) -Ca- (Zr) type alloys and Group 3: Mg-Ca-Zn- (Zr) type alloys

Group 1: Mg- (Zn) -RE system
[22] In the group 1 alloy system, the Mg alloy sheet comprises 0% to less than 3.0% zinc, 0.05% to less than 1.0% rare earth element or a mixture thereof, 0% It contains more than 1.0% calcium, more than 0% and less than 0.3% strontium and the balance Mg and other inevitable impurities. The rare earth element or mixture thereof can include the rare earth elements or mixtures of rare earth elements discussed above. However, in a preferred embodiment, the RE content comprises 0.05% to 1.0% Gd and greater than 0% to 0.3% lanthanum (La). In some embodiments, the Group 1 Mg alloy comprises more than 0.5% but less than 2.0% Zn, 0.05% to 1.0% Gd, 0.05% to 1.0%. Ca, more than 0% to 0.3% strontium (Sr), 0% to 0.3% lanthanum (La) and the balance Mg, and other inevitable impurities.

Group 2: Mg-Zn- (RE) -Ca- (Zr) type alloys:
[23] In the alloy system of Group 2, Mg alloy is more than 0.5% but less than 2.0% Zn, 0.05% to 1.0% Ca, 0% to 1.0% Gd 0.1% to 1.0% Zr, 0% to 0.3% strontium (Sr), 0% to 0.3% lanthanum (La) and the balance Mg, and other inevitable impurities . In addition, preferably the amount of Zn ranges from 0.5% to 2.0%. Furthermore, the amount of Ca is preferably more than 0.1% and less than 1.0%. In addition, it is preferable to contain more than 0.05% and less than 0.7% Gd. Furthermore, the amount of Zr is preferably more than 0.2% and less than 0.7%. In addition, the amount of Sr is preferably less than 0.2%. Furthermore, the content of La is preferably less than 0.2%.

Group 3: Mg-Ca-Zn- (Zr) type alloys.
[24] In the group 3 alloy system, the Mg alloy is more than 0.5% but less than 1.5% Ca, 0.1% to 0.8% Zn, 0% to 1.0% Gd. 0% to 0.7% Al, 0% to 0.3% Sr, 0 to 1.0% Zr, and the balance Mg and other inevitable impurities. In addition, the Ca content is preferably in the range of 0.6% to 1.0%. Furthermore, the amount of Zn is preferably more than 0.3% and less than 0.5%. In addition, the amount of Gd is preferably greater than 0.05%, preferably greater than 0.1% and less than 0.5%. Furthermore, the amount of Al is preferably more than 0.1%, more preferably more than 0.2% and less than 0.5%. In addition, the amount of Sr is preferably less than 0.2%. Furthermore, the amount of Zr is preferably more than 0.2% and less than 0.7%.

  [25] The dilute magnesium alloy sheet is a magnesium alloy having a dilute alloyed content. Therefore, the total amount of alloying elements is less than 3%. The additional alloying additive is detrimental to the formability of the Mg wrought alloy because it causes the formation of second phase particles that can act as nucleation sites for cracks during deformation. It is understood that it may be.

  [26] The dilute magnesium alloy sheet is preferably formed from a magnesium type spreading alloy. In the embodiment, the magnesium type alloy includes Mg-1Zn-0.4Gd-0.2Ca, Mg-1.3Gd, Mg-1Zn-0.5Ca, Mg-2Zn-0.4Gd-0.2Ca, Mg- 2Zn-0.5Ca, Mg-0.8Ca-0.4Zn-0.1Sr-0.5Zr, Mg-0.8Ca-0.4Zn-0.4Gd-0.5Zr, Mg-0.8Ca-0. It is selected from one of 4Zn-0.1Sr-0.4Gd-0.5Zr, Mg-2Zn-0.5Ca-0.5Zr. In a preferred embodiment, the magnesium type alloy is Mg-2Zn-0.5Ca, Mg-2Zn-0.5Ca-0.5Zr or Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0. Selected from one of .5Zr.

  [27] Manganese (Mn) can also be added to both Zr-free and Zr-containing alloys to minimize iron content and further improve corrosion resistance. When present, the amount of Mn is preferably more than 0.05% and less than 0.7%, more preferably more than 0.1% and less than 0.5%.

  [28] Magnesium type alloys preferably contain a minimal amount of incidental impurities. In some embodiments, the magnesium-type alloy includes less than 0.5 wt% incidental impurities, more preferably less than 0.2 wt%. Incidental impurities include Li, Be, Ca, Sr, Ba, Sc, Ti, Hf, Mn, Fe, Cu, Ag, Ni, Cd, Al, Si, Ge, Sn, and Th alone or in combination. May be included in various amounts.

[29] The dilute magnesium type alloy sheet used in the strengthening process can be manufactured, produced or manufactured using any one of a number of known manufacturing methods for manufacturing magnesium sheets. In some embodiments, the dilute magnesium type alloy sheet product comprises:
Preparing a magnesium alloy melt from the magnesium-type alloy;
Casting the magnesium alloy melt into a slab or strip according to a predetermined thickness;
Homogenizing or preheating a cast slab or strip; and
Subsequently, in order to reduce the thickness of the homogenized or preheated slab or strip, the homogenized or preheated slab or strip is hot-rolled at an appropriate temperature to obtain an alloy sheet product of a predetermined thickness. Manufacturing steps and
Annealing the alloy sheet product for a time at an appropriate temperature;
It is manufactured using.

  [30] The magnesium alloy melt can be produced using any suitable method. In many embodiments, the individual elements were mixed and melted in a furnace, such as a high frequency induction melting furnace, in a suitable vessel such as a mild steel crucible to a temperature above the liquidus temperature for this alloy embodiment. In some embodiments, the melt is heated to about 760 ° C. under an argon atmosphere.

  [31] The casting step can include any suitable casting process. For example, the casting step may include casting an ingot or billet. In other embodiments, the casting step may include casting into a sheet or strip. In some embodiments, casting includes casting the magnesium alloy melt directly into one of a chill (DC) caster, a sand mold caster, or a mold caster. For example, the casting step may include using a DC casting billet that is subsequently extruded after preheating to form a slab or strip. In other embodiments, the casting step includes feeding a magnesium alloy melt between the rolls of a twin roll caster to create a strip.

  [32] Homogenization or preheating of the cast slab or strip preferably occurs at a temperature between 300-420 ° C. The actual homogenization temperature depends on the alloy composition. In some embodiments, the cast slab or strip is homogenized or preheated followed by quenching, preferably water quenching. The homogenization or preheating of the cast slab or strip is preferably performed for about 0.25 to 24 hours.

  [33] The homogenized slab or strip is preferably machined into a 5 mm thick strip and then hot rolled. Hot rolling is preferably performed in a temperature range of 300 to 550 ° C, more preferably 350 to 500 ° C. Hot rolling typically results in a total thickness reduction of 60-95%, preferably 70-90%.

  [34] In some embodiments, hot rolling is performed using multiple rolling passes, wherein after each rolling pass, the sheet is at a temperature in the range of 350-500 ° C. before the next rolling. It is reheated with. The sheet is reheated for about 5-20 minutes, preferably 5-10 minutes, preferably. The thickness reduction per pass is preferably about 20%. Thus, the total thickness can be about 80% because the thickness reduction per pass is about 20%.

  [35] After final rolling, the sheet undergoes a final annealing treatment to remove strain accumulated through static recrystallization. The annealing temperature is preferably ± 50 ° C. from the inflection point of the annealing curve obtained for a standard period of 1 hour for the alloy composition. Further, the period for annealing the alloy sheet product is preferably about 1 minute to 24 hours.

[36] The present invention will be described with reference to the accompanying drawings, which illustrate particularly preferred embodiments of the invention.
[001] FIG. 1 is a flow chart illustrating a method of making a magnesium wrought alloy according to the invention, including an experimental test regime. [002] FIG. 2 shows (a) Mg treated with T6 (200 ° C., 30 minutes aging) and T8 (1.5% tensile deformation followed by aging at 200 ° C., 30 minutes) as annealed. -Ten Zn-0.4Gd-0.2Ca, (b) Mg-1.3Gd, and (c) Mg-1Zn-0.5Ca alloy sheet tensile curves. [003] FIG. 3 shows as-annealed T6 (200 ° C., 30 minutes aging) and T8 (1.5% tensile deformation followed by 200 ° C., 30 minutes aging) (a) Mg -2Zn-0.4Gd-0.2Ca, and (b) Mg-2Zn-0.5Ca alloy sheet tensile curves. [004] FIG. 4 shows that as-annealed, treated with T6 (200 ° C., 30 minutes aging) and T8 (2.5% tensile deformation followed by aging at 200 ° C., 30 minutes) (a) Mg -0.8Ca-0.4Zn-0.1Sr-0.5Zr, (b) Mg-0.8Ca-0.4Zn-0.4Gd-0.5Zr, and (c) Mg-0.8Ca-0. 4 provides the tensile curve of 4Zn-0.1Sr-0.4Gd-0.5Zr alloy sheet. [005] FIG. 5 shows the tensile of as-annealed and T8 (1.5% tensile deformation followed by aging at 200 ° C. for 30 minutes) Mg-2Zn-0.5Ca-0.5Zr alloy sheet Provide a curve. [006] FIG. 6 shows various annealing conditions (a) 350 ° C. for 10 minutes, (b) 400 ° C. for 10 minutes, (c) 450 ° C. for 5 minutes, and (d) 500 ° C. for 3 minutes. FIG. 4 provides a tensile curve of Mg-2Zn-0.4Gd-0.2Ca as-treated and treated with T8 (1.5% tensile deformation followed by aging at 200 ° C. for 30 minutes). [007] FIG. 7 shows as-annealed and various aging conditions (a) 150 ° C., 12 hours, (b) 150 ° C., 24 hours, (c) 200 ° C., 30 minutes, (d) 200 ° C. FIG. 6 provides a tensile curve of Mg-1Zn-0.5Ca treated with T8 at 60 minutes and (d) 200 ° C. for 120 minutes. [008] FIG. 8 provides the tensile curve of Mg-1Zn-0.5Ca alloy under T8 treatment. Pre-deformation was introduced by cold rolling under various thickness reductions of 5%, 8% and 10%.

[Detailed description]
[37] We have determined that the strength of dilute magnesium alloy sheets formed from alloys containing small amounts of zinc and calcium / rare earth elements introduces a small amount of plastic deformation into the dilute magnesium alloy sheets, followed by It has been found that it can be improved by using a strengthening method with an aging treatment, and sometimes significantly improved. The discovery of the strain-induced aging strengthening phenomenon of the present invention provides an effective means for strengthening these magnesium alloy sheets having a high degree of ductility and formability and increasing the commercial value of these alloy sheets.

  [38] While not being limited to any one theory, the inventors have compared the strengthening resulting from the processing method or process of the present invention to known age-hardening mechanisms known for magnesium alloys. Therefore, it is considered that they have different curing mechanisms. In the prior art, Mg-5Y-2Nd-2 heavy rare earth-0.4Zr, wt. % (WE54) and Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr, wt. It has been found that plastic deformation of Mg alloys with high alloy concentrations such as% causes precipitation hardening resulting from denser precipitates. However, dilute Mg alloy sheets (which are the subject of the present invention) are reinforced using mechanisms that are likely to be effective pinning of mobile basal plane dislocations by GP zones and solute atoms.

  [39] Although the present invention can be applied to strengthen highly formable magnesium dilute sheet alloys, the present invention provides for Mg- (Zn) -RE, Mg-Zn- (RE) -Ca- It can also be applied to extruded magnesium products made from (Zr) and Mg-Ca-Zn- (Zr) compositions. Exemplary dilute magnesium type alloys include three general alloy systems: Mg- (Zn) -RE system, Mg-Zn- (RE) -Ca- (Zr) system, and Mg-Ca-Zn- It has been found that it is generally classified as (Zr) system. Each alloy sheet of these systems has a tensile stretch or cold temperature at room temperature where the tensile plastic strain will be controlled in the range of more than 0.5% but less than 8%, preferably 1-4%. It can be plastically deformed, such as cold rolling, and the thickness reduction of cold rolling preferably does not exceed 10%. The predeformed magnesium alloy sheet is subsequently subjected to an aging treatment in a temperature range of 80 to 250 ° C., and the aging time is preferably 24 hours or less. Improvements in strength of dilute magnesium alloy sheets between 20-129% (33-139 MPa) have been demonstrated through application of the method of the present invention, as outlined in the following examples.

  [40] FIG. 1 illustrates a flow chart illustrating a method of making a magnesium alloy sheet used in the method of the present invention. As shown in FIG. 1, a dilute magnesium type alloy according to the composition described herein is first prepared in start step 105.

  [41] Following the preparation of the melt, in step 110, the individual alloys are cast using a suitable casting technique. In some embodiments, the casting step may include casting an ingot, billet, bar, square or other shaped body. In other embodiments, the casting step may include casting into a sheet or strip.

  [42] Examples of casting techniques include twin roll casting (TRC), sand casting with or without a chill plate on two sides, casting or DC casting. It is understood that several direct chill (DC) casting methods and equipment suitable for magnesium alloys are known in the art and can be used in the process / method of the present invention. The strip or slab can also be made from a DC cast billet that is subsequently extruded again into the slab or strip using methods and equipment suitable for magnesium alloys known in the art.

  [43] In one embodiment, the alloy was melted and cast using a high frequency induction melting furnace with a mild steel crucible at about 760 ° C. under an argon atmosphere. The steel mold was preheated to about 200 ° C. before casting. The resulting melt was poured into an appropriately sized ingot 30 mm thick x 55 mm wide x 120 mm long.

  [44] Homogenization or preheating is used to reduce interdendritic segregation and compositional differences associated with the casting process. A suitable commercial practice is to select a temperature below the non-equilibrium solid phase, usually 5-10 ° C. Assuming that magnesium, zinc and calcium are the main components of the alloy, the temperature range is 300-500 ° C. depending on the alloy composition. The time required for the homogenization step depends on the size of the cast ingot, billet, strip or slab. A time of 2-4 hours is sufficient for TRC strips, while up to 24 hours are required for sand cast slabs or direct chill cast slabs. The homogenization process is followed by a quenching step, typically a water quenching step.

  [45] For experimental purposes, the homogenized ingot is machined into a 5 mm thick strip. However, it is understood that the strip can be formed using a number of other techniques, as discussed above in the casting step.

  [46] The homogenized ingot, strip or slab is then hot-rolled at step 120 at a suitable temperature. Depending on the casting material, different rolling steps may be used. The breakdown rolling step can be used for alloy slabs produced by sand casting, DC casting or any other type of casting and having a thickness greater than 25 mm. The purpose of this step is to refine and remove the cast structure in addition to reducing the thickness. The temperature of this step depends on the furnace available at the rolling facility, but temperatures between 350-500 ° C are usually used. For alloy strips manufactured by TRC, there is no need for a breakdown rolling step and rolling is performed at a temperature between 250 ° C and 450 ° C. Hot rolling includes the strip passing many times between rollers. After each rolling pass, before the next rolling, the sheet is typically reheated at a temperature in the range of 350-500 ° C. for about 5-10 minutes to raise the temperature before the next pass. A few cold passes with a percent reduction of 10 or 20% per pass may be used as the final rolling or sizing operation. In step 125, the process continues until the final thickness (within the set error range) is obtained. The reduction per pass can be about 20% and the total reduction can be about 85%. After each rolling pass, the sheet was reheated at a temperature in the range of 350-500 ° C. for about 10 minutes before the next rolling. In some embodiments, additional cold rolling was incorporated as the final rolling.

  [47] After the final rolling, with or without a cold rolling step, the sheet is subjected to an annealing treatment at an appropriate temperature and time to remove strain accumulated via static recrystallization in step 130. It was. Annealing is a heat treatment process that is intended to restore ductility to alloys that have been significantly strain hardened by rolling. There are three stages of annealing heat treatment—recovery, recrystallization, and grain growth. During recovery, the physical properties of the alloy, such as conductivity, are restored, while during recrystallization, the cold-worked structure is replaced by a new set of strain-free particles. Recrystallization is recognized by metallographic methods and can be confirmed by a decrease in hardness or strength and an increase in ductility. If new strain free particles are heated above the temperature required for recrystallization, particle growth occurs and results in a significant loss of strength and should be avoided. The recrystallization temperature depends inter alia on the alloy composition, the initial particle size and the preceding deformation and is therefore not a constant temperature. For practical purposes, the temperature is defined as the temperature at which a highly strain-hardened (cold-worked) alloy will completely recrystallize within one hour.

  [48] The optimal annealing temperature and conditions for each alloy are to determine the approximate temperature at which the alloy is exposed to various temperatures for 1 hour, then the hardness is measured and recrystallization is complete and grain growth begins. It is specified by creating an annealing curve. This temperature may also be specified as the inflection point of the hardness-anneal temperature curve. This technique makes it possible to obtain the optimum temperature easily and reasonably accurately.

  [49] The annealed strip was then quenched with a suitable medium, such as water.

  [50] The formed magnesium alloy sheet is strengthened using a two-step strengthening process.

  [51] First, the formed magnesium alloy sheet is plastically deformed using a pre-deformation step 132 that includes a tensile drawing or cold rolling process at room temperature. In either case, the applied tensile plastic strain will be greater than 0.5% but less than 8%. If cold rolling is used, the thickness change (decrease) will not exceed 10%. This plastic deformation forms a predeformed magnesium alloy sheet.

  [52] Second, the pre-deformed one or more magnesium alloy sheets are aged 135 at a temperature in the range of 80-250 ° C. for at least 1 minute. Aging typically occurs in a temperature controlled environment. Suitable environments include liquid baths such as furnaces or oil baths. An aging treatment typically takes at least 1 minute to 24 hours, although most embodiments are 12 hours or less.

  [53] A series of experiments was conducted to test the basic properties (strength including yield strength) of the embodiment of the alloy described as a dilute magnesium alloy sheet product and for the formed dilute magnesium alloy sheet product. It was done to prove the strengthening properties of the method.

  [54] Several alloy compositions according to the present invention and sheets formed therefrom were formed and tested in these experiments. Table 1 summarizes the composition of each of the diluted magnesium alloy sheet compositions tested.

  [55] Each sheet of the alloy composition was manufactured using the method described above. In these experiments, the individual elements were mixed and melted in a high frequency induction melting furnace using a mild steel crucible at about 760 ° C. under an argon atmosphere. The alloy melt was poured into a steel mold preheated to about 200 ° C. The homogenization treatment was performed at a temperature in the range of 300-420 ° C. depending on the alloy composition. The homogenized ingot was machined into a 5 mm thick strip and then hot rolled at a temperature in the range of 350-500 ° C. The thickness reduction per pass was about 20%, and the total thickness reduction was about 85%. After each rolling pass, the sheet was reheated at a temperature in the range of 350-500 ° C. for about 10 minutes before the next rolling. Optionally, cold rolling was employed as the final rolling. After final rolling, the sheet (with or without cold rolling) was subjected to an annealing treatment to remove the accumulated strain through static recrystallization.

  [56] Following annealing, each sample was subjected to a plastic deformation (in the pre-deformation step) followed by a strengthening treatment according to the method of the invention involving an aging treatment. The pre-deformation step was performed by tensile drawing at room temperature or by cold rolling. The aging treatment was performed in an oil bath, where the aging temperature was set between 80 ° C. and 250 ° C., and the aging time preferably did not exceed 12 hours.

[57] The strain-induced aging response of both the initially formed dilute magnesium alloy sheet and the tempered sheet (ie, pre-deformed sheet aged) is the Instron 4505 tensile with a strain rate of 10 ⁻³ / s. Measured using test. The thickness of each tensile sample was about 0.7-1 mm, and the gauge distance was about 10 mm.

[58] Three sets of samples were made for testing.
A dilute magnesium alloy sheet that was annealed (named O), ie, as formed initially following the annealing process.
An initially formed dilute magnesium alloy sheet that has been annealed and aged (named T6), ie, annealed, and then processed using an aging treatment. No pre-deformation step is performed on this sample. These samples were prepared for comparative purposes to separate from pre-deformation and to quantify the aging effects of dilute magnesium alloy sheets. And Annealed and strain-aged alloy sheet (designated T8)-a dilute magnesium alloy sheet following the pre-deformation step and aging treatment according to the present invention.

  [59] For ease of representation in the following examples, annealed, annealed and aged, and annealed and strain aged alloy sheets were respectively O (annealed), T6 (Annealed and aged) and T8 (annealed and strain aged alloy sheet).

  [60] Detailed alloy compositions, thermomechanical process parameters, and aging and strain aging processes for the corresponding alloy sheets are shown in Tables 1 and 2, respectively.

[61]

[62]

Example 1: Strain-induced aging strengthening of Mg- (Zn) -RE and Mg-Zn- (RE) -Ca-Zr type alloy sheets
[001] Sheets 1-8 shown in Table 2 received O, T6 and T8 treatments, and sheet 9 received O and T8 treatments. The results of these treatments are summarized in Table 3. In addition, as-annealed T6 (200 ° C., 30 minutes aging) and T8 (1.5% tensile deformation followed by 200 ° C., 30 minutes aging) treatment (a) Mg— Fig. 2 shows tensile curves of 1Zn-0.4Gd-0.2Ca (sheet 1), (b) Mg-1.3Gd (sheet 2), and (c) Mg-1Zn-0.5Ca (sheet 3) alloy sheets. Shown in

[63]

  [64] From these results, for the Mg-Zn-RE type alloy sheet (sheet 1) sample represented by Mg-1Zn-0.4Gd-0.2Ca, the aging treatment (T6) is It turns out that it did not bring any increase. However, induction of 1.5% tensile plastic deformation at room temperature, followed by aging treatment (T8), could cause an increase in strength of 55 MPa, with an increment of about 61%. However, for the Mg-RE binary alloy sheet (sheet 2) alloy represented by Mg-1.3Gd, the T6 treatment does not give any improvement in strength, and the T8 treatment also has a strength of about 38 MPa. Was increased (increment 38%). In the case where calcium completely replaced the rare earth element, ie Mg-1Zn-0.5Ca (sheet 3), T6 treatment increased the yield strength from 106 MPa to 125 MPa. On the other hand, the T8 treatment that caused a substantial increase in strength increment of 70 MPa increased the yield strength from 106 MPa to 176 MPa. This intensity increment is much larger than the intensity increment provided by the T6 process.

  [65] Overall, the T8 treatment (deformation plus aging) of the present invention adds small amounts of zinc, calcium and the rare earth element gadolinium, regardless of whether the T6 treatment (aging alone) results in age hardening. It can be concluded that a considerable increase in strength can be given.

Example 2: Effect of zinc content on strain-induced aging strengthening phenomenon
[66] The results for the Mg-2Zn-0.4Gd-0.2Ca (sheet 4) and (b) Mg-2Zn-0.5Ca (sheet 5) alloy sheets shown in Table 3 are for the alloys tested. The zinc content has been shown to have a decisive influence on the degree of strain-induced aging strengthening.

  [67] If the zinc content is increased from 1% to 2% and the gadolinium and calcium concentrations are maintained at 0.4% and 0.2%, respectively, the strength increment from T8 treatment is from 55 MPa. The pressure rose to 67 MPa. Indeed, when the zinc concentration was increased to 2%, even T6 treatment also resulted in an increase in strength of 23 MPa.

  [68] Further, when the zinc content was increased from 1% to 2% and the calcium concentration was maintained at 0.5%, the T8 treatment caused a substantial increase in strength of 90 MPa, which It was an 85% increment compared to the yield strength in the finished state.

  [69] The above is also illustrated in FIG. 3, which shows that as-annealed, T6 (200 ° C., 30 minutes aging) and T8 (1.5% tensile deformation followed by 200 ° C. Of the treated (a) Mg-2Zn-0.4Gd-0.2Ca (sheet 4) and (b) Mg-2Zn-0.5Ca (sheet 5) alloy sheets Provides a tensile curve.

Example 3: Strain-induced aging strengthening response of Mg-Ca-Zn- (Zr) type alloy sheet
[70] Mg-0.8Ca-0.4Zn-0.1Sr-0.5Zr (sheet 6), Mg-0.8Ca-0.4Zn-0.4Gd-0.5Zr (sheet) shown in Table 3 7), and Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr (sheet 8) alloy sheet results show that the strain of Mg-Ca-Zn- (Zr) type alloy sheet Provides an induced aging enhancement response. FIG. 4 also shows the strain-induced aging strengthening response of the Mg—Ca—Zn— (Zr) alloy system.

  [71] The results show that the T6 treatment was Mg-0.8Ca-0.4Zn-0.1Sr-0.5Zr (sheet 6), Mg-0.8Ca-0.4Zn-0.4Gd-0.5Zr ( Demonstrated that the sheet 7) and Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr (sheet 8) alloy caused 42 MPa, 37 MPa and 31 MPa strength increments, respectively. To do. On the other hand, the T8 treatment resulted in higher strength increments of about 88 MPa, 80 MPa and 76 MPa, respectively.

Example 4: Strain-induced aging strengthening response of Mg-Zn-Ca-Zr type alloy sheet
[72] The strain-induced aging strengthening response of the Mg-Zn-Ca-Zr type alloy sheet was examined by the results of the Mg-2Zn-0.5Ca-0.5Zr (sheet 9) alloy sheet (Table 3).

  [73] FIG. 5 shows Mg-2Zn-0.5Ca-0.5Zr (as-annealed and T8 (aged at 200 ° C. for 30 minutes following 1.5% tensile deformation) treatment). Sheet 9) Provides the tensile curve of the alloy sheet. These curves show that the addition of 0.5% Zr to Mg-2Zn-0.5Ca effectively refined the grain size, so that the strength of the annealed state could be increased accordingly. To demonstrate. The yield strength of as-annealed Mg-2Zn-0.5Ca-0.5Zr is about 182 MPa, and the yield strength of the T8-treated one reaches about 234 MPa, which is the result of all the reported dilute Mg- It was the highest among the Zn type sheet compositions.

Example 5: Effect of annealing parameters on strain-induced aging enhancement response
[74] An Mg-2Zn-0.4Gd-0.2Ca alloy sheet (sheet 10) was used to investigate the effect of annealing parameters on strain aging characteristics. The following annealing parameters were examined.
350 ° C for 10 minutes,
400 ° C for 10 minutes,
450 ° C for 5 minutes and 500 ° C for 3 minutes

  [75] The results of these experiments are shown in Table 4.

[76]

  [77] The results showed that low temperature annealing, eg, 350 ° C. annealing, gave the highest yield strength (163 MPa), but this condition resulted in a low strength increment of 33 MPa with T8 treatment, as shown in FIG. Indicates that As the annealing temperature is increased, the yield strength of the as-annealed sheet gradually decreases from 102 MPa at 400 ° C. for 10 minutes to 92 MPa at 500 ° C. for 3 minutes, however, the strength increase due to T8 treatment is approximately 79 It appears to be highly stable at ~ 82 MPa.

Example 6: Effect of aging parameters on strain-induced aging enhancement response
[78] The effect of aging parameters on the strain-induced aging strengthening response of Mg-1Zn-0.5Ca (sheet 3) alloy sheet in T8 treatment is the same predeformation at 1.5% plastic strain for each sample And while maintaining the annealing conditions (400 ° C. for 10 minutes), the aging parameters (time and temperature) were varied. The various experimental conditions and results of these experiments are shown in Table 5.

[79]

  [80] The results show that when the Mg-1Zn-0.5Ca alloy sheet is aged at 150 ° C., the strength increment remains a similar value despite the different aging times, as shown in FIG. It shows that it is stably maintained at about 68 to 70 MPa. When the Mg-1Zn-0.5Ca alloy sheet was aged at 200 ° C., it took only 30 minutes to reach a 69 MPa strength increment. With increasing aging time, the strength increase of the alloy sheet decreased slightly. When the strain-induced aging time reached about 120 minutes, the strength increment decreased to 66 MPa.

Example 7: Effect of cold rolling as a means of pre-deformation on strain-induced aging strengthening response
[81] The effectiveness of cold rolling on the strain-induced strengthening response to T8 treatment is shown in Mg-1Zn-0.5Ca under various cold rolling reductions, ie 5%, 8% and 10%. A sheet sample (sheet 11) was tested and subsequently examined by performing an aging treatment and a tensile test.

  [82] The test results are shown in FIG.

[83]

  [84] These results show that as the cold rolling reduction increased from 5% to 10%, the strength increase of the alloy sheet treated by the T8 treatment changed from 49 MPa to 139 MPa.

Conclusion
[85] Numerous experimental results show that the strength of Mg- (Zn) -RE, Mg-Zn- (RE) -Ca- (Zr) and Mg-Ca-Zn- (Zr) type dilute sheet alloys It shows that it has increased significantly under the aging treatment (T8 treatment in the above-mentioned embodiment). Even though some alloy sheets can be age hardened (eg, the T6 treatment of the examples), these strength increments obtained with age hardening alone are far greater than those caused by the strain-induced strengthening treatment of the present invention. Small.

[86] Thus, compared to conventional magnesium alloy strengthening methods, the method of the present invention provides the following five advantageous differences.
1. Dilute Alloy Additive—The present invention reinforces dilute magnesium alloy sheets, ie sheets with <3 wt% alloying elements. This has not been reported before. It is unexpected that such a dilute alloy Mg sheet has a plastic strain-induced age hardening phenomenon.
2. Significant improvement in strength-The magnitude of strengthening in the above examples is as high as 129%, which is unexpected, especially for such dilute alloy sheets.
3. Small plastic strain-It is unexpected that such a small tensile strain, such as the 2% tensile plastic strain in the above example, can also cause a significant improvement in strength.
4). Different mechanisms of hardening-As detailed above, the strengthening mechanism is an effective pinning of mobile basal plane dislocations by GP zones and solute atoms, in contrast to precipitation hardening reported in the prior art. Probability is high. And 5. Easy processability—The alloy sheets encompassed by the present invention can be easily produced from an as-cast ingot by hot rolling. Mg alloys having a strong age hardening effect, such as those mentioned in the prior art, are generally very difficult to process. Conventional alloys can only be hot extruded or rolled with very little reduction in thickness and are therefore very difficult to produce into rolled sheets.

  [87] The inventors have described that the above features make the alloy sheet of the present invention particularly suitable for a range of existing production techniques including extrusion, forging and twin roll casting, particularly in vehicle or automotive applications. I think.

  [88] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications as fall within the spirit and scope of the invention.

  [89] The terms “comprise,” “comprises,” “comprised,” or “comprising” are used herein (including the claims). As used herein, these terms specify the presence of a presented feature, whole number / integers, step or component, but one or more other features, whole number / integer, It is not to be construed as excluding the presence of steps, ingredients, or groups thereof.

  [90] Future patent applications may be filed in Australia or abroad based on this application, or may be priority claims from this application. It is understood that the following provisional claims are provided by way of example and do not limit the scope of what might be claimed in any such future application. At a later date, features may be added to or deleted from the provisional claims to further define or redefine one or more of the present inventions.

Claims (23)

> 0-3.0 wt% of Zn,> 0 to 1.5 wt% of Ca, 0 to 1.0 wt% of Zr, 0 to 1.3 wt% of rare earth or 0 to 1.3 wt% there in the rare earth element mixture, from 0 to 0.3 wt% of Sr, 0 to 0.7 wt% of Al, the step of providing a magnesium alloy sheet the balance being Mg and other unavoidable impurities or Rana Ru magnesium alloy The total weight percent of alloying elements is less than 3 weight percent; and
The magnesium alloy sheet, the tensile plastic strain exceeds 0.5% by plastically deformed to be less than 8%, a step of forming a magnesium alloy sheet which has been pre-deformed, plastic deformation, at room temperature A step realized by at least one of tensile drawing or cold rolling performed, and the thickness reduction of the magnesium alloy sheet due to the cold rolling does not exceed 10% ;
Aging the pre-deformed magnesium alloy sheet at a temperature range of 80-250 ° C. for at least 1 minute;
Including
Thereby forming a reinforced magnesium alloy sheet,
How to strengthen the magnesium alloy sheet. Tensile plastic strain is controlled in the range of 1-4% A method according to claim 1. Temperature range of aging treatment is between 100 and 250 ° C., The method of claim 1. Aging treatment is carried out in air or in oil, The method of claim 1. Aging is 24 hours or less, The method of claim 1. The method of claim 1, wherein the aging treatment is at least 1 minute. The method of claim 1 , wherein the aging treatment is between 5 minutes and 12 hours. Increase in the strength of the reinforcing magnesium alloy sheet for magnesium alloy sheet is at least 10 Pa%, The method of claim 1. Increase in the strength of the reinforcing magnesium alloy sheet for magnesium alloy sheet is between 20 Pa% and 100 Pa%, The method of claim 8. Increase in the strength of the reinforcing magnesium alloy sheet for magnesium alloy sheet is at least 20 MPa, Process according to claim 1. Increase in the strength of the reinforcing magnesium alloy sheet for magnesium alloy sheet is between 33MPa and 139MPa, The method of claim 10. Magnesium alloy comprises 0.1 to 3.0 wt% of Zn, the method of claim 1. Magnesium alloy comprises 0.05 to 1.5 wt% of Ca, the method according to claim 1. Magnesium alloy,
> 0-3.0 wt% Zn,
> 0 to 1.0 wt% Ca,
0.05 to 1.0 wt% rare earth element or 0.05 to 1.0 wt% rare earth element mixture,
0-0.3 wt% of Sr, as well as Mg and other unavoidable impurities <br/> or Rana Ru Mg- (Zn) -RE type alloy remainder The method of claim 1. The magnesium alloy comprises a rare earth element mixture,
Rare earth mixture comprises gadolinium and yttrium, the method according to claim 1. The magnesium alloy comprises a rare earth element mixture,
The method of claim 1 , wherein the rare earth element mixture comprises gadolinium or yttrium , and La. The method according to claim 1 , wherein the magnesium alloy contains only gadolinium as a rare earth element. Magnesium alloy,
> 0-3.0 wt% Zn ,
> 0 to 1.5 wt% Ca ,
0-1.0 wt% Zr ,
0 to 1.0 wt% Gd ,
0-0.3 wt% Sr ,
0-0.3 wt% La ,
0 to 0.7 wt% of Al, and Ru Rana or Mg and other unavoidable impurities <br/> balance The method of claim 1. Magnesium alloy,
0.5 to 2.0% by weight of Zn,
0.05 to 1.0 wt% Ca ,
0-1.0 wt% Zr ,
0 to 1.0 wt% Gd ,
0-0.3 wt% Sr ,
0-0.3 wt% of La, as well as Mg and other unavoidable impurities <br/> or Rana Ru Mg-Zn- (Gd) -Ca- ( Zr) type alloy remainder to claim 1 The method described. Magnesium alloy,
0.5-1.5 wt% Ca ,
0.1 to 0.8 wt% Zn ,
0-1.0 wt% Zr ,
0 to 1.0 wt% Gd ,
0-0.7 wt% Al ,
0-0.3 wt% of Sr, as well as Mg and other unavoidable impurities <br/> or Rana Ru Mg-Ca-Zn- (Zr) type alloys remainder The method of claim 1. Magnesium alloy further containing 0.05 to 0.7 wt% of Mn, the method according to claim 1. Magnesium alloy with the balance Mg-1 wt% Zn-0.4 wt% Gd-0.2 wt% Ca, the balance Mg-1 wt% Zn-0.5 wt% Ca, the balance Mg-2 wt% Zn-0.4 wt% Gd-0.2 wt% Ca, balance Mg-2 wt% Zn-0.5 wt% Ca, balance Mg-0.8 wt % % Ca-0.4 wt% Zn-0.1 wt% Sr-0.5 wt% Zr, balance Mg-0.8 wt% Ca-0.4 wt% Zn-0. 4 wt% Gd-0.5 wt% Zr, balance Mg-0.8 wt% Ca-0.4 wt% Zn-0.1 wt% Sr-0.4 wt% Gd- The method of claim 1 , wherein the method is selected from one of 0.5 wt% Zr. Forming a magnesium alloy sheet,
Preparing a magnesium alloy melt from a magnesium type alloy;
Casting the magnesium alloy melt into a slab or strip according to a predetermined thickness;
Homogenizing or preheating the cast slab or strip;
Subsequently, in order to reduce the thickness of the homogenized slab or strip, the homogenized or preheated slab or strip is hot-rolled at an appropriate temperature to produce an alloy sheet product of a predetermined thickness. And steps to
Annealing the alloy sheet product for a time at an appropriate temperature;
The method of claim 1 comprising: